Dielectric optical waveguide

ABSTRACT

A method of fabricating dielectric optical waveguides comprises the steps of: (1) fabricating a single or double heterostructure from the GaAs-AlGaAs system preferably by liquid phase epitaxy or molecular beam epitaxy; (2) forming a native oxide layer on the top surface of the heterostructure by anodization in H2O2; (3) removing a portion of the oxide layer to form a mask and hence to define the waveguide shape in the direction of light propagation; and (4) forming a mesa-like structure with optically flat side walls by etching at a slow rate in Br2-CH3OH. After step (4) two alternative techniques leading to structurally different waveguides may be followed. In one technique, an AlGaAs layer is epitaxially grown over the mesa to form a two dimensional waveguide. In the other technique, the edges of the active region of an AlGaAs double heterostructure are differentially etched in a neutral solution of H2O2. The latter step is particularly useful in the fabrication of active devices because the resulting structure is self-masking, thereby facilitating the formation of electrical contacts.

United Sta Logaiuet al.

DIELECTRIC OPTICAL WAVEGUIDE Inventors: Ralph Andre Logan, Morristown;

Bertram Schwartz, Westfield; Joseph Charles Tracy, Jr., Bernardsville;William Wiegmann, Middlesex, all of NJ.

Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

Filed: Dec. 26, 1973 Appl. No.: 427,914

Related US. Application Data Division of Ser. No. 291,937, Sept. 25,1972 Pat. No. 3,833,435.

Assignee:

U.S. Cl. 350/96 WG; 331/945 H Int. Cl. G02b 5/14 Field of Search 350/96WG; 331/945 H References Cited UNITED STATES PATENTS 9/1969 Stern 350/96WG UX 7/1972 Paoli et a1 33l/94.5 H

9/1973 Thompson 2/1974 Miller 350/96 WGX GROW EPI AlGaAs 51 May 13, 1975Primary Examiner-John K. Corbin Attorney, Agent, or Firm-M. .1. Urbano[57] ABSTRACT A method of fabricating dielectric optical waveguidescomprises the steps of: l) fabricating a single or doubleheterostructure from the GaAs-AlGaAs system preferably by liquid phaseepitaxy or molecular beam epitaxy; (2) forming a native oxide layer onthe top surface of the heterostructure by anodization in H 0 (3)removing a portion of the oxide layer to form a mask and hence to definethe waveguide shape in the direction of light propagation; and (4)forming a mesa-like structure with optically flat side walls by etchingat a slow rate in Br Cl-l OH. After step (4) two alternative techniquesleading to structurally different waveguides may be followed. in onetechnique, an AlGaAs layer is epitaxially grown over the mesa to form atwo dimensional waveguide. In the other technique, the edges of theactive region of an AlGaAs double heterostructure are differentiallyetched in a neutral solution of H 0 The latter step is particularlyuseful in the fabrication of active devices because the resultingstructure is self-masking, thereby facilitating the formation ofelectrical contacts.

9 Claims, 8 Drawing Figures PMENTEU HAY I 3l975 SHEET 1G? 2 NATIVE OXIDEFORM DOUBLE HETEROSTRUCTURE AND NATWE v OXIDE LAYER FIG. 2

DEFINE STRIPE BY PHOTOLlTHOGRAPHY F/G. 5A

DIFFERENTIALLY ON GDAS SUBSTRATE EPI GROW AlGuAs LAYER 8T THEN GGASLAYER TO FORM SH EPI GROW ANOTHER AIGGAS LAYER TO FORM DH FORM NATIVEOXIDE ON TOP SURFACE 4 OF SH OR DH FORM OXIDE STRIPE BY PHOTOLITHOGRAPHYPOLISH ETCH WITH Br2CH3OH TO FORM MESA WITH OPTICALLY FLAT SIDE wALLsLPE I MBE DIFFERENTIALLY ETCH PORTION REGROWTH? I REGROWTH? OF GGASLAYER WITH NEUTRAL H202 T0 FORM TwD DIMENSIONAL WAVEGUIDE REMOVE DxIDESTRIPE I IF ACTIvE DEvICE, DEPOSIT EPI CRDw AIGCIAS LAYER ELECTRICALCDNTACTS To FORM TWO DIMENSIONAL DIRECTLY BECAUSE DEVICE WAVEGUIDE ISSELF MASKING IF ACTIvE DEvICE, FORM ELECTRICAL CDNTACTS BYPHOTOLITHOGRAPHIC MASKING DIELECTRYC OPTICAL WAVEGUIDE This applicationis a division of parent application Ser. No. 291,937, filed Sept. 25,1972, now US. Pat. No. 3,833.435 and was concurrently filed withapplication Ser. No. 427,915 which is also a division of said parentapplication.

BACKGROUND OF THE INVENTION This invention relates to dielectric opticalwaveguides and more particularly to the fabrication of such wave guidesfrom the GaAs-AlGaAs system.

It has been proposed by S. E. Miller in the Bell Sysrem TechnicalJournal, Vol. 48, pages 2059 et seq. (1969) that optical signals can beprocessed using a form of integrated circuitry similar to that used inmicrowave technology. Such circuitry would find important applicationsin high-capacity optical communications systems and optical computers.The circuits could contain narrow dielectric light (wave) guides whichwould serve as the basis for both active components (e.g., modulators,detectors and light sources) as well as passive components (e.g.,couplers, filters and interconnections).

A typical dielectric optical waveguide comprises an elongated core ofdielectric material surrounded by a medium having a lower index ofrefraction, e.g., a core of GaAs surrounded by AlGaAs. When oneconsiders a cross-section ofsuch a waveguide perpendicular to itsoptical axis (i.e,, the z-direction), it is apparent that such astructure confines light in two dimensions (i.e., the .r and ydirections). Hereinafter this type of structure will be referred to as atwo-dimensional wa e guide. The degree of confinement is a function ofthe refractive index difference between the core and its surroundingmedium, and the optical loss per unit length is a function of thequality ofthe interface therebetween. With respect to the interfaces,.1. E. Goell et al. in Applied Physics Lerlers, Vol. 21, pages 72 et seq(1972) have pointed out that the smoothness of the walls of thewaveguide is an important consideration in the fabrication of adielectric waveguide. Excessive scattering loss results when thewaveguide has rough walls. Thus. for example, in a waveguide having theshape of a rectangular parallelepiped, one of the more perplexingproblems that has plagued the prior art is the inability to controladequately the smoothness of the guide walls to a tolerance of afraction of an optical wavelength over a dimension of about 5wavelengths (see, D. Marcuse, Bell System Technical Journal, Vol. 48,pp. 3187 et seq. (1969)). In a GaAs dielectric waveguide, for example. arelevant wavelength in the semiconductor is 1 0.25 pm. Consequently, thesmoothness dimension of the dielectric waveguide walls should preferablybe controlled to a tolerance of less than about 0.1 A or 250 angstromsover a dimension of about 1.25 am.

The advent of the double heterostructure laser raised hopes that apractical two-dimensional dielectric wave guide might become a reality.The double heterostructure, as described by M. B. Panish et al. inScientific American, ol. 224. page 32 et seq (1971), typically comprisesa 1.0 ,um thick GaAs layer sandwiched between relatively thicker layersof AlGaAs. Heterojunctions formed at the interfaces with the GaAs layerform a dielectric waveguide which guides light in the directionperpendicular to the plane of the layers, i.e., the

LII

growth plane in the case of either liquid phase epitaxy (LPE) ormolecular beam epitaxy (VIBE) fabrication. However, since noheterojuncti ns are fo med perpen dicular to the growth plane. light isnot guided in the direction parallel to the growth plane. i.e., thedielectric waveguide of Panish et al is one-dimensional Although thework of Parrish et al. was greeted by the technical world withconsiderable enthusiasm, no one has taught a practical way to utilizedouble heterostructure concepts to produce a two-dimensional dielectricwaveguide in which, for example, a substantially rectangularparallclepiped core of GaAs is surrounded on four sides by AlGaAs and inwhich the dimensions of the guide are controlled to within a few hundredangstroms.

SUMMARY OF THE INVENTION In accordance with an illustrative embodimentofour invention, a technique for fabricating a twodimensional dielectricwaveguide comprises the steps of: (l) fabricating a single or doubleheterostructure from he GaAs-AlGaAs system. Preferably theheterostructure is fabricated by a liquid phase epitaxial (LPE) growthtechnique taught by M. B. Panish et al. in Melallurgical Transactions,Vol. 2, pages 795-801 (1971) or by a molecular beam epitaxy (MBE) growthtechnique taught by J. R. Arthur, Jr. in US. Pat. No. 3,615,931 (Case 3)issued on Oct. 26, 1971, and as further taught by A. Y, Cho in copendingpatent application Ser. No. 127,926 (Case 2) filed on Mar. 25, 1971 (nowUS. Pat. No. 3,751,310 issued on Aug. 7, 1973) and assigned to theassignee hereof. Of the two, MBE may be preferred since more precisecontrol of layer thicknesses is attainable; (2) forming a native oxidelayer on the top major surface of the heterostructure by anodization inH O: in accordance with the teachings of B. Schwartz in copendingapplication Ser. No. 292,127 (Case 13) tiled on even date herewith'nowUS. Pat. No. 3,798,139 issued Mar. 19, 1974 and assigned to the assigneehereof. The use ofa native oxide is preferred over a conventional oxidesuch as SiO The latter is relatively more difficult to fabricate,generally requiring an evaporation scheme, and, in addition, presentssome difficulties in adhering to the GaAs- AlGaAs layers. The nativeoxide, of course, being a part of the top layer itself. presents noproblems of ad hesion and moreover is relatively simple to fabricate;(3) removing a portion of the native oxide layer by standardphotolithographic techniques in order to define from the remaining oxidelayer a mask having a predetermined shape, e.g., a stripe; and (4)forming a mesa-like structure by bringing the heterostructure intocontact with a Br CH;,OH solution which slowly etches away the portionsof the GaAs-AlGaAs layers not protected by the native oxide mask.lmportantly, the bromine concentration is carefully controlled so thatthe etching rate is relatively slow, e.g., l-3 rim/hr. in this range notonly are the side walls of the mesa (i.e., of the waveguide) madeoptically flat over an extended length, but also the cross-sectionalshape of the mesa remains substantially constant along the extendedlength (i.e.. the etching is uniform along the length).

The terms mesa and mesa-like structure will be used interchangeablyhereinafter to mean a waveguide in which the cross section, takenperpendicular to the direction of light propagation therein, has theapproximate shape of a truncated triangle. This definition may be inconflict with the conventional definition of a mesa which has thegeneral shape of a truncated cone.

At this point in our inventive fabrication technique either of twoalternative approaches may be followed. One approach includes the stepsof: epitaxially growing by MBE or LPE an AlGaAs layer over the mesastructure. thereby forming another pair of heterojunctions at theinterfaces with the edges of GaAS layer. Thus, the GaAs layer (i.e., thecore) is surrounded on all four sides with the smaller index ofrefraction material, AlGaAs; and (6) if the structure is to be utilizedas an active device (e.g., a junction laser), making electrical contactsto the device utilizing an appropriate masking technique. It should benoted, however, that inasmuch as an active device includes an activeregion such as a p-n junction, the AlGaAs layer grown in step (5) mustbe at least semi-insulating in order that the active region not beshort-circuited.

In order to avoid the necessity of growing such a semi-insulating layerand to eliminate the mask alignment steps which naturally arise in themaking of electrical contacts to such a device, we propose analternative approach which results in a self-masking dielectricwaveguide. More specifically, assume that initially a doubleheterostructure, comprising an Al Ga, As middle layer sandwiched betweenouter layers of A1,. Ga, ,As and AI,Ga, As, y x and z, was fabricated instep (1). Then, after step (4) above the following steps are performed:(5) bringing the double heterostructure mesa into contact with a neutralsolution of H 0 preferably agitating the solution while in contact withthe heterostructure, as taught by I. C. Dyment, R. A. Logan, and B.Schwartz in copending application Ser. No. 291,941 (Case 6-19-15) filedconcurrently herewith now U.S. Pat. No. 3,810,391, issued on Apr. 2,1974, and assigned to the assignee hereof. This solution etchesdifferentially the middle layer at a faster rate than the adjacent Al,Ga,As and Al Ga,-,As layers of the heterostructure as long as y x and z.As a result, the middle layer is etched inwardly from its edges leavinga central core of Al Ga ,,As bounded on its edges by air and on its topand bottom by outer Al- GaAs layers which overhang the middle layer. Theresultant pedestal-like structure has the significant advantage that itis self-masking. That is to say, where the middle layer is an activeregion of an active device, the formation of electrical contacts simplyrequires the additional step of: (6') depositing a suitable conductor(e.g., metal) over the entire top surface of the mesa. Because the outerAlGaAs layers overhang the middle layer and form an air gaptherebetween, the deposited conductor will be bifurcated at the air gapand will not short circuit the active region. Photolithographic techniques, with attendant mask alignment problems, are not required.

BRIEF DESCRIPTION OF THE DRAWING The invention, together with itsvarious features and advantages, can be easily understood from thefollowing more detailed description taken in conjunction with theaccompanying drawing, in which:

FIGS. 1 to 3 show the structural changes followingvarious steps in thefabrication of a double heterostructure mesa in accordance with anillustrative embodiment of our invention;

FIG. 4A shows an illustrative embodiment of a di electric waveguideformed by growing an AIGaAs epitaxial layer over the mesa of FIG. 3;

FIG. 4B shows an illustrative embodiment of another dielectric waveguidewhich results from the initial fabrication of a single heterostructureinstead of a double heterostructure;

FIG. 5A shows an illustrative dielectric waveguide in accordance withstill another embodiment of our invention in which the mesa of FIG. 3has been exposed to an etchant which differentially etches the AIGaAslayers;

FIG. 5B demonstrates the self-masking feature of the structure of FIG.4B; and

FIG. 6 is a flow chart of the principal fabrication steps in accordancewith an illustrative embodiment of our invention.

DETAILED DESCRIPTION An illustrative embodiment of our invention willnow be described with concurrent reference being made to the flow chartof FIG. 6 and to the schematic structures of FIGS. 1 5. The latterfigures depict the sequential structural changes in illustrativedielectric waveguides after each principal step in our inventivetechnique is completed. Of course, for simplicity and clarity ofexplanation FIGS. 1 5 are not necessarily drawn to scale.

Turning now to FIG. 1, there is shown a muIti-layered device comprisinga GaAs substrate 10 on which have been epitaxially grown the followinglayers in the sequential order recited: an Al Ga, ,As layer 12, x 0; anAl Ga, ,,As layer 14, 0 =2 y x; and an Al Ga, As layer 16, 2 y. Theinterfaces 13 and 15 between layer 14 and layers 12 and 16, form a pairof heterojunctions which will ultimately serve to confine light in theydirection, i.e., perpendicular to the growth plane. For a symmetricwaveguide structure, of course, the atomic percent of Al in layers 12and 16 should be the same, i.e., x 2. Typically the substrate 10 isn-type GaAs with the end face 11 being a (011) cleavage plane and thetop surface 17 being a growth plane.

The double heterostructure of FIG. 1 may ultimately form either anactive device or a passive device depending upon the carrierconcentrations in the various layers and upon the operationalenvironment in which the device is utilized. Thus, for example, for useas a junction laser, layers 12, 14, and 16 typically have n-p-p typeconductivity, respectively, thereby forming a p-n heterojunction atinterface 13 and a p-p heterojunction at interface 15. Under forwardbias, and when mounted on a suitable heat sink and in an opticalresonator, this type of laser has been successfully operated on acontinuous wave basis at room temperature as discussed by M B. Panish etal. in Scientific American, supra. Alternatively, the device mayfunction as a phase or amplitude modulator if layer 14 is made to be acompensated, high resistivity layer as taught by F. K. Rein- 'hart incopending application Ser. No. 193,286 (Case 2) filed on Oct. 28, 1971(now U.S. Pat. No. 3,748,597, issued on July 24, 1973) and assigned tothe assignee hereof. As a passive device, on the other hand, thisstructure may be utilized simply as a transmission line, i.e., adielectric waveguide, in which the light is guided in layer 14. In allof the foregoing active and passive devices, however, it is desirable toconfine the light not only in the y-direction perpendicular to thegrowth plane but also in the x-direction parallel to the growth plane,assuming that light is propagating in the zdirection.

To this end the subsequent steps in our inventive technique will bedescribed in terms of a stripe geometry structure although it is to beunderstood that more complicated geometric shapes of the dielectricwaveguide are readily achieved by means of suitable masks in aphotolithographic technique. In order to define such a stripe, the nextstep in our technique is to form a native oxide layer 18 (FIG. 1) on theAlGaAs top layer 16. The term native oxide as used herein means an oxideformed from the constituent elements of the underlying layer rather thanfrom a foreign element not included in the molecular compound of theunderlying layer. Thus, for example, we do not prefer to utilize a SiOlayer which is relatively more complicated to fabricate and which tendsnot to adhere to the AlGaAs top layer 16. With these problems in mind,we have found that a native oxide formed by the anodization scheme of B.Schwartz in copending application Ser. No. 292,127 (Case 13, supra) ispreferred. Briefly, in this technique the double heterostructure of FIG.1 is placed in an electrolyte bath illustratively comprising H 0 (30percent) and H 0 (70 percent). The double heterostructure is made theanode whereas a noble metal such as platinum is made the cathode. Theelectrolyte bath is typically buffered with phosphoric acid to a pH of2.0 and a source of about 100 volts DC is connected between the anodeand cathode. After about minutes a native oxide layer is grown having athickness of about 1,850 angstroms. Next, the double heterostructure ofFIG. I is removed from the bath and air dried by heating, for example,to 100 Centigrade for 1 hour and then to 250 Centigrade for 2 hours. Ingeneral, a suitable pH range is about I to 6 and a suitable voltagerange is about 5 to 175 volts.

After drying is completed, portions of the native oxide layer 18 areremoved by standard photolithographic techniques in order to defineillustratively an elongated oxide stripe 20 as shown in FIG. 2. Asdescribed hereinafter, this stripe will be utilized to form a mesastructure and ultimately a two-dimensional waveguide. It should be notedthat the oxide stripe 20 is highly irregular from the standpoint ofoptical smoothness. More specifically, we have found that the oxidestripe 20 typically has along its edges 22 and 24 peak-to-peakvariations of 1pm in its width dimension (w) which occur with 1 amperiodicity in the zdirection along the stripe. One would not expectsuch an irregular stripe to produce the smoothness required of a goodquality dielectric optical waveguide; e.g., in GaAs a smoothness ofabout 0.1 A or 250 angstroms over a length of about 1.25 pm. Moreover,as discussed by E. G. Spencer et al. in J. Vacuum Sc. & Tech., Vol. 8,pp. 852-70, at S63 1971), one would normally not expect conventionaletching technology to be able to reduce these irregularities to therequired optical smoothness.

We have discovered, however, that a Br CH Ol-I etchant, of sufficientdilution to produce a relatively slow etching rate, etches away theGaAs-AlGaAs layers not protected by oxide stripe 20 and importantly doestwo things: l it produces optically flat mesa side walls and (2) itetches uniformly so that the cross-sectional shape of the mesa issubstantially constant over its length. More specifically, a Br CH OHsolution containing approximately 0.5 to L0 parts bromine per 1,000 byvolume produces an etching rate of about l-3 um per hour. At this ratethe etchant acts as a polish so that along the top edges 26 and 28 (FIG.3) of the mesa the amplitude of the irregularities is reduced by afactor of at least 10 and their periodicity is increased by a factor ofat least 100. Thus, this relatively slow Br CH OH etchant effectivelyproduces mesa side walls having a high degree of optical smoothness asrequired for dielectric optical waveguides.

Incidentally, the etchant undercuts the oxide stripe 20 as shown in FIG.3 leaving a portion of stripe 20 which overhangs mesa edges 26 and 28.The overhang, however, has been found to produce no difficulties in thesubsequent fabrication steps.

In addition, the etchant has the desirable property that it does notattack, i.e., dissolve. the native oxide stripe 20 an essentialrequirement if the top surface of the mesa, and hence its shape, is tobe preserved during the etching step.

When the growth plane of the epitaxial layers is and the cleavage planeis (01 l we found, in addition, that the slanted side walls of the mesaare {l l l planes. The obliqueness of the side walls is an importantfeature if molecular beam epitaxy is to be utilized to subsequently growan AlGaAs layer over the mesa as will be described more fullyhereinafter.

At this point in our inventive technique two alternative approaches maybe followed depending upon the ultimate structure desired. In one typeof structure shown in FIG. 4A, an Al Ga, ,,As layer 30, q v, isepitaxially grown over the mesa structure of FIG. 3 by molecular beamepitaxy, liquid phase epitaxy, or any other suitable technique. Layer 30forms another pair of heterojunctions at the interfaces 32 and 34 withlayer 14 and thereby serves to confine light, in the xdirection. Thus,the four heterojunctions at interfaces l3, I5, 32 and 34 bound the lightguide core (layer 14) and confine light in both the x and y directions.A two dimensional waveguide is thereby formed.

It should also be noted that layer 30 serves to passivate the sides ofthe waveguide structure by preventing contaminants from entering fromeither the top or side surfaces.

Where the growth of layer 30 is by means of LPE, it may be desirable toleave oxide stripe 20 (FIG. 3) on the top layer 16 during the growthprocess so that the liquid solution utilized to grow layer 30 does notwet and dissolve the top surface of layer 16 and thereby deteriorate theoptical quality of the dielectric waveguide. In this case layer 30 wouldnot grow on the oxide mask but only on the side walls of the mesa. Onthe other hand, where layer 30 is to be grown by MBE, then the oxidestripe 20 may first be removed. Most mineral acids and common bases willserve this purpose, e.g., HCI in a solution of one part concentrated HCland one part H O. Utilizing this technique, particularly good quality,smooth layers 30 hai 'e been grown by MBE on sidewalls and the top ofthe mesa.

Moreover, whereas some etchants tend to produce vertical side walls onthe mesa, we have found that Br CH OH in the above-specifiedconcentration preferentially etches the lll planes provided that theepitaxial growth plane is (100). The {111} planes form an angle of about53 degrees with the horizontal, i.e., the x-direction. When utilizingMBE to grow layer 30, the presence of such slanted side walls isparticularly advantageous to avoid shadowing of the side walls from themolecular beam which might occur if the side walls were more nearlyvertical. Shadowing, of course, could result in incomplete coverage ofthe side walls and hence a partial or complete failure of the structureto guide light in the v\'--direction.

The shadowing problem may be further alleviated by an optional step inwhich. after removing the native oxide, the edges of layer 16 can berounded off by etching in bromine methanol. A bromine concentration aspreviously described will initially etch the edges at a faster rate thanthe more central portions of the layer.

Where the structure of FIG. 4A is to be utilized as an active device,such as a junction laser or phase modulator, it is necessary that thelast grown AlGaAs layer 30 be at least semi-insulating in order that theactive region (e.g., the p-n junction) of the device not beshortcircuited. For an active device, therefore, the next step in theprocedure would be to form electrical contacts to the substrate 10 andto the AlGaAs layer 16, illustratively by evaporation. Of course, beforecontact to layer 16 can be effected an appropriate photolithographicmasking and etching technique would be utilized to expose apredetermined portion of the top surface of layer 16. The lattertechnique could readily utilize the native oxide masking and brominemethanol etching procedures previously described.

From the foregoing description of the sequential steps in thefabrication of a double heterostructure dielectric waveguide (FIG. 4A),it is at once apparent to one skilled in the art that initially theprocedure could have begun with the fabrication of a single heterostructure only, in which case the AlGaAs layer 16 of FIG. 1 would not befabricated. In all other respects, however, the fabrication procedurewould follow the steps previously described and the resultant structurewould appear as shown in FIG. 4B. In both FIGS. 4A and 4B, Al,,Ga ,,Aslayer 14 forms an elongated dielectric core surrounded on four sides byAlGaAs layers 30 and 12 (FIG. 4B) and layers 30, 12 and 16 (FIG. 4A).The amount of aluminum in the core is less than that of the surroundinglayers so that the core has a higher index of refraction. Thus, lightpropagating in the z-direction in layer 14 is confined thereto by twopairs of heterojunctions which form a two-dimensional dielectricwaveguide.

Where the structures of FIG. 4A and 4B are active devices, difficultiesmay arise in making the last grown AlGaAs layer 30 semi-insulatingand/or in aligning photolithographic masks for making electricalcontacts. Under such circumstances, a preferred approach in accordancewith another illustrative embodiment of our invention is to begin withthe mesa structure of FIG. 3 and to differentially etch away the outerportions of layer 14 to produce the pedestal-like structure shown inFIG. 5A. To insure this result, the amount of aluminum in layer 14 mustbe less than that in layers 12 and 16 precisely the situation whichobtains in a double heterostructure, i.e., y x and z. The need for lessaluminum in layer 14 arises from the fact that a substantially neutralsolution of H 0 acts as a differential etchant, i.e., it etchesAl,,Ga,-,,As at a faster rate as y decreases. This phenomenon is taughtin Dyment-Logan-Schwartz, copending application Ser. No. 291,941, Case6-19-15, supra. In FIG. 2 of that application etching rate versuspercent aluminum in AlGaAs is plotted for a solution of 30 percent H 0in LJI water buffered with NI-l OI-I to a pH of 7.05. For example, ify Oin Al ,,Ga, ,,As, (i.e., GaAs) then the etching rate is about 6 um/hr,whereas ify 0.1 the rate rapidly drops off to about 0.6 um/h. Thus. in2: DH having a GaAs middle layer sandwiched between Al Ga A5 layers, themiddle layer will etch at a rate about I0 times faster than the AlGaAslayers.

During the etching process oxide platelets are formed on the etchedsurfaces. It is preferred, therefore, that the solution and/or structurebe agitated during the etching step. In one useful technique which wehave utilized, the structures were waxed with apiezon-w-wax to a 2inchdiameter quartz disk which was placed in the bottom of a beakercontaining the etching bath. The beaker was maintained about 30 offvertical and was rotated at about 60 rpm during the etching step.

As a result, we have achieved highly uniform, smooth etching of thelayer 14. In addition, we have found that the etchant dissolves thenative oxide. The combined differential etching of AlGaAs anddissolution of the native oxide produces the structure of FIG. SA, wherea growth plane was used, the interior edges 36 and 38 of layer 14 arerespectively parallel to the preferential I 1 1 I} etching planes 40 and42 which form the side walls of the mesa.

Although an H 0 solution with a pH of 7.05 is preferred, we havedetermined that a useful pH range is approximately 6 to 8. Below a pH ofabout 6 the solution acts as an oxidant rather than an etchant. Incontrast, above a pH of about 8 etching proceeds so rapidly thatundersirable pitting of the etched surfaces may occur.

In an illustrative example, we fabricated on a GaAs substrate a doubleheterostructure having I am thick GaAs middle layer sandwiched betweenrelatively thicker layers (e.g. 3-6pm) of AI Ga As. The DH was initiallyabout 8 mm wide, 12 mm long and 0.4 mm thick (including the substrate).After etching with bromine methanol, a mesa was formed having a 12 umwidth along its top surface. After differentially etching in an agitatedsolution of H 0 (pH of 7.05), the width of the middle layer was reducedfrom about 12 um to about 1 am whereas the width of the contiguous Al-GaAs layers was practically unaffected.

The waveguide structure of FIG. SA has several use ful features. First,in a DH junction laser the narrowed middle layer 14 would serve not onlyto enhance current confinement but also transverse mode control.Secondly, and importantly, the structure is self-masking. That is tosay, even though the layer 14 might form the active region of an activedevice (e.g., include a p-n junction) no photolithographic masking isrequired when making electrical contact to layer 16. More spe cifically,as shown in FIG. 58, contact to layer 16 is simply made by evaporatingor otherwise forming a metallic contact 44 over the entire top surfaceof the mesa structure. Because layers 12 and 16 overhang (i.e., extendlaterally beyond) layer 14, and form an air gap therebetween, thedeposited contact 44 is bifurcated at the air gap and will notshort-circuit the active region 14. Illustratively, contact 44 is agold-chromium alloy evaporated onto a p-AlGaAs layer 16 and contact 46is a tin-platinum alloy evaporated onto n-GaAs substrate 10. Connectionto an external circuit is illustratively made by means of bonded wires48 and 50.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of our invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, the doubleheterostructure of FIG. 1 may take on various structural configurationssuch as, for example, a double-double heterostructure of the typedescribed by l. Hayashi in US. Pat. No. 3,691,476 (Case 5) issued onSept. 12. 1972 and assigned to the assignee hereof, or a modified doubleheterostructure (which includes a p-n homojunction between a pair ofcommon-conductivity-type heterojunctions) as described by L. A. D 'Asaroet al. in copending application Ser. No. 203,709 (Case 11-12) filed onDec. l, 1971 (now abandoned) and assigned to the assignee hereof.

What is claimed is:

l. A dielectric optical waveguide comprising:

a GaAs substrate having first and second major surfaces,

a multilayered structure comprising the following layers epitaxiallygrown on said first major surface in the order recited: a Al Ga As firstlayer at least one A1,, Ga ,,As middle layer, and a Al Ga AS thirdlayer, y x and z;

the width of said middle layer measured parallel to said first majorsurface being less than the corresponding width of said first and thirdlayers so that said latter layers have portions which overhang the edgesof the said middle layer.

2. The waveguide of claim 1 wherein said middle layer forms the activeregion of an active device and said overhanging portions form air gapstherebetween, and further including a first electrical contact formedover the surface of said waveguide which includes said third layer, saidfirst contact being bifurcated at said air gaps so that said activeregion is not short-circuited by said first contact, and a secondelectrical contact formed on said second major surface of saidsubstrate.

3. The waveguide of claim 1 wherein said multilayered structure has, ina plane perpendicular to the direction of light propagationtherethrough, a mesa-like cross-section which is made substantiallyuniform over an extended length of said waveguide, and which has sidewalls which are made optically flat, by etching said structure at a slowrate in a bromine methanol solution containing approximately 0.5 to 1.0parts bromine per 10 1,000 by volume.

4. The waveguide of claim 3 wherein the width of said middle layer ismade smaller than that of said first and third layers by bringing saidstructure into contact with a solution of H 0 in water buffered to a pHin the range of about 6 to 8, said solution differentially etch saidmiddle layer at a faster rate than said first and third layers, so thatsaid first and third layers overhang said middle layer and create airgaps therebetween.

5. The waveguide of claim 4 wherein said solution has a pH of about7.05.

6. The waveguide ofclaim 14 wherein y 0 and said middle layer comprisesGaAs.

7. The waveguide of claim 4 wherein said first major surface of saidGaAs substrate is parallel to a crystallographic plane, the plane ofsaid cross-section is parallel to the (011) cleavage plane of GaAs, andsaid side walls are parallel to {111} crystallographic planes which arepreferentially etched by said solution of bromine methanol.

8. A dielectric optical waveguide having along its length a mesa-likecross-section in a plane perpendicular to the direction of lightpropagation therethrough. comprising a GaAs substrate,

a heterostructure epitaxially grown on said substrate, saidheterostructure including at least an A1,. Ga, As first layer, .r 0,grown on said substrate and an Al Ga As second layer, 0 s y .r, grown onsaid first layer and being adapted for the propagation of lighttherethrough, said heterostructure having the shape of a mesa formed bybringing said heterostructure into contact with a solution of brominemethanol containing about 0.05 to 0.l percent bromine, said solutionbeing effective to produce optically flat side walls on said mesa and asubstantially uniform cross-section along the length of said waveguide,and

an Al Ga As, q y, third layer epitaxially grown on said heterostructureso that said second layer is bounded on four sides by AlGaAs layershaving more aluminum therein, thereby to form a twodimensionalwaveguide.

9. The waveguide of claim 8 including an Al,Ga, ,As

fourth layer, z grown on said second layer, said Al Ga, As third layerbeing grown on said Al,Ga ,As

fourth layer and on the side walls of said mesa.

1. A DIELECTRIC OPTICAL WAVEGUIDE COMPRISING: A GAAS SUBSTRATE HAVINGFIRST AND SECOND MAJOR SURFACES, A MULTILAYERED STRUCTURE COMPRISING THEFOLLOWING LAYERS EPITAXIALLY GROWN ON SAID FIRST MAJOR SURFACE IN THEORDER RECITED: A AL2GA1-2AS FIRST LAYER AT LEAST ONE ALYGA1-GAS MIDDLELAYER, AND A AL2GA1-2AS THIRD LAYER, Y<X AND Z? THE WIDTH OF SAID MIDDLELAYER MEASURED PARALLEL TO SAID FIRST MAJOR SURFACE, BEING LESS THAN THECORRESPONDING WIDTH OF SAID FIRST AND THIRD LAYERS SO THAT SAID LATTERLAYERS HAVE PORTIONS WHICH OVERHANG THE EDGES OF THE SAID MIDDLE LAYER.2. The waveguide of claim 1 wherein said middle layer forms the activeregion of an active device and said overhanging portions form air gapstherebetween, and further including a first electrical contact formedover the surface of said waveguide which includes said third layer, saidfirst contact being bifurcated at said air gaps so that said activeregion is not short-circuited by said first contact, and a secondelectrical contact formed on said second major surface of saidsubstrate.
 3. The waveguide of claim 1 wherein said multilayeredstructure has, in a plane perpendicular to the direction of lightpropagation therethrough, a mesa-like cross-section which is madesubstantially uniform over an extended length of said waveguide, andwhich has side walls which are made optically flat, by etching saidstructure at a slow rate in a bromine methanol solution containingapproximately 0.5 to 1.0 parts bromine per 1, 000 by volume.
 4. Thewaveguide of claim 3 wherein the width of said middle layer is madesmaller than that of said first and third layers by bringing saidstructure into contact with a solution of H2O2 in water buffered to a pHin the range of about 6 to 8, said solution differentially etch saidmiddle layer at a faster rate than said first and third layers, so thatsaid first and third layers overhang said middle layer and create airgaps therebetween.
 5. The waveguide of claim 4 wherein said solution hasa pH of about 7.05.
 6. The waveguide of claim 14 wherein y 0 and saidmiddle layer comprises GaAs.
 7. The waveguide of claim 4 wherein saidfirst major surface of said GaAs substrate is parallel to a (100)crystallographic plane, the plane of said cross-section is parallel tothe (011) cleavage plane of GaAs, and said side walls are parallel to(111) crystallographic planes which are preferentially etched by saidsolution of bromine methanol.
 8. A dielectric optical waveguide havingalong its length a mesa-like cross-section in a plane perpendicular tothe direction of light propagation therethrough, comprising a GaAssubstrate, a heterostructure epitaxially grown on said substrate, saidheterostructure including at least an AlxGa1 xAs first layer, x > 0,grown on said substrate and an AlyGa1 yAs second layer, 0 < or = y < x,grown on said first layer and being adapted for the propagation of lighttherethrough, said heterostructure having the shape of a mesa formed bybringing said heterostructure into contact with a solution of brominemethanol containing about 0.05 to 0.1 percent bromine, said solutionbeing effective to produce optically flat side walls on said mesa and asubstantially uniform cross-section along the length of said waveguide,and an AlqGa1 qAs, q > y, third layer epitaxially grown on saidheterostructure so that said second layer is bounded on four sides byAlGaAs layers having more aluminum therein, thereby to form atwo-dimensional waveguide.
 9. The waveguide of claim 8 including anAlzGa1 zAs fourth layer, z > y, grown on said second layer, said AlqGa1qAs third layer being grown on said AlzGa1 zAs fourth layer and on theside walls of said mesa.